1. Air Pollution
Air pollution exists when the atmosphere contains an objectionable concentration of one or more deleterious substances. A substance may be deleterious because it is an irritant. For example, it may be irritating to the eyes, nose, skin, or mucus membranes of the respiratory system, or may even cause difficulties in breathing.
In other cases pollutants may be toxic, causing chronic or acute reversible or irreversible physiological damage to humankind, animals, or vegetation. Ultimately this includes serious disease processes, such as black lung disease, brown lung disease, and malignancies. In some case pollutants may even lead to extermination of species.
Other effects of air pollution include deterioration of materials of construction and of structures, or the creation of areas where certain plant species cannot survive.
The atmosphere has always served as a gaseous and vapor waste dispersion medium and disposal area. Waste materials, in the form of fogs, dispersions, particulates, combustion products, and the like, are introduced into the air, become widely dispersed, diluted, hydrated, and even photochemically reacted. Some of these products then settle to the ground or are removed by precipitation. The precipitation and particulates may themselves be harmful. Other products may stay dispersed in the atmosphere, building up, and causing further problems.
Human activities, especially industrial activities, have added both new pollutants to the atmosphere and increased the background concentrations of pollutants to a very high degree. Synthetics, acidic combustion products, as SO.sub.2, and photodissociable synthetic molecules, as chlorofluorocarbons, have been added to the ecological load by human industrial activities.
2. Ambient Air Quality Standards
Ambient air quality standards are designed to protect the atmosphere from air pollution.
Air pollution has three components: (1) source emissions, (2) atmospheric transport and dispersal, and (3) receipt by a receptor. The degree of an air pollution problem depends upon the interaction of these components.
Time is an important factor in each of the three components of air pollution. In the case of the source, for example, emissions are continuous from a power plant but intermittent from a dry cleaning plant. In the case of atmospheric transport, inversions, stationary fronts, and atmospheric high pressure systems which remain over a region for several days all reduce dispersion. This reduced dispersion can produce air pollution episodes during which pollutant concentrations build up. For receptors, duration of exposure to pollutants is important. Humans may survive large dosages of some pollutants and then eliminate the pollutant from their bodies. However, other pollutants can accumulate in the body over time, and are only respirated or excreted with great difficulty, eventually causing illness or death. For these reasons, standards for air pollutant concentrations specify an averaging period of from minutes to a year.
In the case of emission standards, as distinguished from time-averaged concentration standards, some emission standards are also set with time limitations. For example, plume opacity standards which allow a certain period of time for start-up and maintenance operations.
If pollutants are allowed to enter the air, nationwide and worldwide emissions over a period of years may cause an increase in the background concentration of that pollutant. This may create a condition in which new normal concentrations, which are higher then old normal concentrations, either interfere with normal atmospheric processes, as is the case with the "Green-house Effect," the polar holes in the ozone layer, and acid rain, or the new "background" of the pollutant may itself be directly deleterious to human health. Ambient air quality standards are intended to limit the concentration of pollutants.
3. Sources
Sources are the pollutant emitters. As a first approximation, sources can be treated individually as point sources. Point sources include each stack at a power plant, or petroleum refinery. Alternatively, or additionally, sources may be treated collectively as area sources, for example, all of the automobile exhausts in a metropolitan area.
Pollutants can be emitted from sources as gases or aerosols, that is, mist, smoke, soot, fume, and dust. Once in the atmosphere, the primary pollutant may undergo transformation into a secondary pollutant, for example, the oxidation of sulfur dioxide to sulfates or the ultraviolet scission of chlorofluorocarbons to free radicals.
4. Fugitive Sources
Emissions from a source generally enter the atmosphere from a distinct stack or exhaust pipe, as an exhaust pipe or stack. While mobile sources, that is, automobiles, and large stationary sources ranging from power plant and industrial stacks down to residential chimneys emit most of the man-made pollutants, pollutants also originate from a very large number of very small, diverse locations around a building, a piece of equipment, a dusty road, or a storage pile These are called fugitive emissions.
Fugitive industrial emissions are another source of pollutants. This is especially true in the case of solvents, halocarbon vapors, cleaning fluids, refrigerants, and the like, as well as radioactive emissions. Fugitive emissions may escape into the atmosphere through windows, vents, and doors rather than through regular exhaust systems. Rosebrook et al. "The Measurement of Fugitive Hydrocarbon Emissions from Selected Sources in Petroleum Refineries," Proceedings, Annual Meeting--Air Pollution Control Association, 1978, 71st, Volume 3, Paper 78-36.4, 15 pp., CA:92:168224z, present relative fugitive emission data for hydrocarbon sources, such as valves, flanges, pump seals, compressor seals, and drains at four typical petroleum refineries. They report that compressor seals have the highest average leak rates, while leakage from flanges are practically insignificant. They also report that because of their large numbers, the highest total leakage is from valves.
Hanzevack et al, "Fugitive Hydrocarbon Emissions--Measurement and Data Analysis Methods," U.S. Environmental Protection Agency Office of Research and Development, EPA 1978, EPA-600/2-78-199, Proceedings: Symposium/Workshop On Petroleum Refinery Emissions; PB-287 900, 41-54, CA:93: 172919t, presents data on valve fugitive emissions at a petrochemical plant. Fugitive emissions are correlated with maintenance, and is reported to show emission reduction through improved maintenance.
While each individual fugitive emission is likely to be small, the total outflow from fugitive emissions is likely to be quite large. The identification and measurement of fugitive emissions is required for both emission inventorying and subsequent pollution control efforts.
5. Fugitive Sources and the Emission Inventory
An important requirement for any air pollution control program is a comprehensive emission source inventory. This inventory identifies sources, source locations, pollutant types, and quantities of emissions throughout the relevant areas. A further aspect of the emission source inventory, especially if mathematical modeling of pollutant dispersion is contemplated, includes such engineering information as source heights, diameters, capacities, and pollutant stream constituents, exit velocities, temperatures, and concentrations. These data are used to model the plume emanating from the source.
Data for the emission inventory may be gathered from plant surveys, consumption summaries, production information, and stack-sampling reports.
6. Fugitive Halocarbon Emissions
Halocarbon emissions, including fluorocarbon, chlorocarbon, and chlorofluorocarbon emissions, occur from a number of sources, including, among others, purification system vents, product-loading vents, hoods, vents, stacks, joints, seals, fittings, gaskets, and valves, including relief valves. In practice, fugitive emissions, once identified and located, are minimized by, among other expedients, repair, replacement, or even enclosure of the emission sources and collection of the emissions.
Other expedients for the control of halocarbon fugitive emissions include: installation of primary and redundant incineration facilities for both point-source and collected fugitive emissions; installation of scrubbing and neutralization or recovery units in conjunction with the incinerators; installation of closed-process sewers, collection systems and larger or redundant wastewater strippers; replacement of single mechanical seals on pumps and agitators with double mechanical seals; leak-detection systems and portable monitors; enclosed sampling and analytical systems; and vapor-recovery systems for halocarbon loading, unloading, and equipment clearing.
7. Measurement of Fugitive Emissions
Pollution measurements are divided into two categories: ambient measurements and source measurements. Pollution measurements, whether source measurements or ambient measurements, often require detection and measurement in the ppmv to ppbv (parts per million by volume to parts per billion by volume) range. This is especially true to determine trends. As a general rule source concentrations range from tenths of a percent to a few hundred ppmv, and ambient concentrations are lower.
In metropolitan areas of high population density and in highly integrated manufacturing facilities, extensive sampling networks have been established. These networks use continuous monitors installed at strategic locations the data transmitted to a common repository or processor.
Ambient air sampling is typically used to:
(1) establish and operate a pollution incident alert network,
(2) locating the source of an undesirable pollutant,
(3) obtaining permanent sampling records for legal action,
(4) monitor a stationary emission source,
(5) model changes in the system, and
(6) establish long term trends.
Source sampling differs markedly from ambient sampling and has unique problems and techniques which are distinct and different from ambient sampling. Source gas temperatures may be high and the raw source gas may contain high concentrations of water vapor or entrained mist, dust, or other interfering substances. The high temperatures may require specialized techniques and equipment, while the particulates and gases may deposit on, or be absorbed in, or otherwise interfere with the sampling probes before reaching the sensor.
A primary objective of source sampling is to prove compliance with regulations. Other objectives of source sampling are to provide emission data and emission data trends, and in this way to determine the need for maintenance of process or control equipment, and measure product loss. In the case of large stationary sources continuous sampling is utilized, and this sampling may be under the control of a central processor. However, in the case of an industrial complex with miles of process piping with instrumentation, and many valves, fittings, elbows, and flanges, sampling is typically infrequent, occasional, or intermittent. This is especially so in the case of sampling fugitive emissions.
Gaseous fugitive emissions, like all gaseous pollutants, are detected by their chemical nature. In the case of halocarbon and hydrocarbon gas fugitive emissions techniques such as gas chromatography, flame ionization, photo ionization, infrared techniques, and the like are typically used.
Typically, volatile organic compound detection systems operate on photoionization and infrared principles in the concentration ranges of from less then 100 ppmv to more then 10,000 ppmv. Volatile organic compound detectors are described, for example, by Menzies, K. T. et al, "Evaluation of Potential VOC Screening Instruments," U.S. Environ. Prot. Agency, Res. Dev., (Rep), EPA, 1983, EPA-600/9-83-003, Incineration Treat. Hazard. Waste: Proc. Annual Res. Symp., 8th; PB83-210450, pp. 143-158, CA:100:161189s. Other volatile organic compound detectors include flame ionization detectors, as described, for example in Summers, C. H. et al., k"Use of Portable Instrumentation For the Monitoring of Fugitive Organic Emissions from Hazardous Waste Incinerators," Report, 1984, EPA-600/2-84-103, CA:102:11494q, and Lawes, H., "Detecting, Locating, and Measuring of Organic Chemical Fugitive Emissions," Int. Env. Saf., 1980 (Apr.), pp. 7-10, CA:93:209408d.
For fugitive emission sampling purposes, the measurement may require the determination of the temperature, concentration, and characteristics of the gas contaminants. Adequate data collection also requires the mass rates of emission of each contaminant. This requires that concentration and volumetric flow rate data be taken.
Fugitive industrial emissions are typically sampled in the air space around fittings, pumps, valves, flanges, hoods, small stacks, and the like. Most commonly fugitive industrial emissions are non-condensable hydrocarbons and halocarbons, relatively free of particulates. In the sampling of noncondensable gases free of particulates, the gases are extracted by a single-point grab sample, a single-point integrated sample, or a multipoint integrated sample. Typically, the sampling probe is fabricated of stainless steel, borosilicate, quartz glass, aluminum, copper, or Teflon. The sample is drawn into the sample probe by a one-way squeeze bulb that is attached to the probe. A glass or Pyrex-wool filter may be inserted in the probe tip to remove any unwanted particulates.
In more elaborate systems a pump first sucks the gas through a cold trap. This dries the gas. Then the gas is drawn through a rotameter to measure flow rate, and into the analytical apparatus.
Fugitive emission sampling and monitoring is by its very nature a laborious, labor intensive, time consuming process. Because of the large number of sources, that is, every pipe, valve, flange, gasket, hood, stack, and vent, it is not practical to provide constant, on-line, real time monitoring, with either scanned data capture or interrupt driven data capture. To the contrary, it has been necessary to take gas sampling equipment to each potential fugitive gas emissions source, individually identify the source, for example by entering the source identifier on a keyboard or numeric pad, and individually sample the source. Source sample results would then be manually entered, for example, again using a keyboard or numeric pad. The possibility of human error is high, especially in inclement weather or at relatively inaccessible sites in a factory.
Thus, a need exists for a simple system that can identify a fugitive emission to a specific source, store the source identifier and the emission rate, and correlate the collected data.